Temperature affects the rates of ALL reactions.
The rule of thumb is that for every 10C increase in temperature, the reaction rate doubles. So a reaction at 30C will be twice as fast as one at 20C.
Also, for precipitation, temperature affects solubility, so that affects the concentration that precipitation begins.
Rule of thumb, solids are more soluble in water as the temperature increases. (The opposite is true for gases dissolved in water.)

Kirk, you were right! I re-ran the precipitation-experiment at 20C (instead of 30C), and sure enough, slight cloudiness became visible after 40 min instead of 20 min. That means at 24C, it'll be 30 minutes, so this rule applies to the concentrate:
"If using hard tap-water, develop promptly, making sure development is finished within 30 minutes after mixing."

But I'd like to do better than that, so I'll investigate chelating agents.

I also waited one hour and then developed a test-strip to see what harm this cloudiness does. I'll scan it in 1-2 days.

@Relayer: I haven't forgotten your request to test your modified formula. I'd like to in a few days as time permits.